Quite sensitive and small infrared (IR) sensors which operate on the principles of a thermopile are now available. Such sensors provide a small electrical signal which varies with the relative strength of the IR radiation impinging on them, and can be used to measure the temperature or change in temperature of an object on which a sensor of this type is focussed. The most sensitive of these sensors can detect differences in temperature of a few thousandths of a degree Celsius in the object from which the IR radiation emanates. Using common photolithographic processes, such sensors can easily be fabricated in situ in a matrix or array, each sensor forming one of the pixels in the array.
It is known that most objects emit amounts and frequencies of infrared radiation which differ based on the emissivity, angle of the surface to the viewer, and temperature of the body itself. The variations in this radiation when allowed to impinge on an array of IR sensors produces corresponding differences in the signals from the individual sensor elements in the array. The individual signals from the sensors in the array can therefore collectively encode an image of the field of view from which the IR radiation emanates. Typically, the individual output signals from the sensors are scanned in some sequential manner to form a composite signal encoding the image and changes in it in real time. In this way, such an array can form the IR radiation-sensitive, image-forming element of a camera which produces images based on the infrared radiation emanating from the field of view. The signal can be used to form a visible image in a display which accurately represents the spatial relationship of objects in the field. Such encoding of an image has been done for many decades in the imaging of visible light from a field of view as in television technology.
A recently developed preferred design for IR sensors depending on a thermoelectric mechanism to provide the signal voltage output, has thin layers of conductive materials of various types and insulating material which are deposited in appropriate patterns on a silicon sensor substrate using well-known photolithographic techniques. Thermoelectric junctions are formed by overlapping conductors during the deposition. Such sensors will be called hereafter microbridge sensors. The junctions of these microbridge sensors are of two kinds, sensor junctions and reference junctions. The reference junctions are in close thermal contact with the substrate. Each sensor junction is within a small, discrete, area which overlays a pit or depression formed in the sensor substrate, and is of an area conforming to the footprint of the sensor junction. In cross section, these sensors look much like a bridge spanning a valley, hence the term "microbridge". The pits provide a measure of thermal isolation from the substrate for their associated sensor junctions. Thus, changing IR radiation impinging on both the sensor and reference junctions causes the temperature of the sensor junction to change more rapidly than does the reference junction, resulting in a temperature differential between the junctions which generates a signal. These known photolithographic techniques allow individual sensors to be easily fabricated in an array so as to allow imaging of the IR radiation in a field of view. Leads from the elements forming the junctions are led to electronic circuitry which may be formed in layers below the sensor substrate. This circuitry scans and amplifies the signals from the individual sensors to provide a signal which may be used to reproduce the field of view in a way analogous to that of television.
For maximum sensitivity microbridge sensors may be maintained in a low pressure gas atmosphere or in a vacuum by virtue of reduced heat transfer between the sensing junction and the substrate, but this requires a hermetically sealed enclosure which adds cost and reduces reliability. It is also possible to use a less tightly sealed enclosure containing air or other gas at or near atmospheric pressure, at the cost of less sensitivity. One should realize that these microbridge sensors are designed to produce a usable signal with but a few hundredths or thousandths of a degree Celsius temperature differential between the sensing and reference junctions.
One desirable application for these sensors is in arrays for forming images of relatively low contrast scenes or fields of view, such as may arise indoors in occupied rooms. In such fields of view, the inanimate, non-heat producing objects are all very nearly at the same temperature. Distinguishing such objects by use of IR imaging requires very sensitive sensors. The types of microbridge IR sensors formed according to today's technology cannot provide the high quality signals, i.e. resolve contrasts in impinging radiation adequately so as to clearly distinguish the typical variations in IR radiation in low contrast fields of view unless the total area of each individual sensor pixel is larger than a certain minimum area. Typically, a 6 mil.times.6 mil (0.15 cm.times.0.15 cm) or equivalent area is required for vacuum-packaged sensors. Even larger areas are required for sensors operating in gas-filled packages. Sensors having such areas are too large to reliably fabricate using current processes. In essence, the span necessary for the bridge which supports the sensing junction is too great for reliable fabrication and adequate resistance to shock and vibration.